Composite materials comprising fibers maintained in a matrix are currently widely used for making parts in many fields of industry, in particular in the field of aeronautics, including for structural parts, i.e., in view of supporting significant efforts on the order of magnitude of their structural resistance during their use.
There are many composite materials, the most widespread in the field of parts designed for structures, such as those used in aeronautical constructions, being composed of more or less long fibers of inorganic or organic (glass, carbon, aramid . . . ) materials contained in a matrix formed by a hard organic resin, which, at least one stage of the method for making the part, is sufficiently fluid to make possible the making of the shapes of the parts [sic—Tr.] before being hardened, for example, by polymerization.
To combine lightness and stiffness, some components such as structural panels with large dimensions, such as the panel 10 shown in FIG. 1a, are made by means of a relatively thin skin 12, whose stiffness is reinforced by stiffeners 13a, 13b assembled on one or both surfaces of said skin. The stiffeners may have various shapes, for example, the so-called Ω shapes of FIG. 1a and FIG. 1c, or Z, U and L shapes, for example.
The structural parts of this type made of composite material must meet the manufacturing tolerances and strict quality requirements.
They are most often made by means of molds which guarantee, by an appropriate use, the qualities sought for the part, in particular the dimensional characteristics, geometry and resistance.
A particular difficulty encountered during the making of certain shapes of the part, such as the stiffened panel 10 of FIG. 1a, is linked with the existence of closed hollow volumes 13a, 13b which generally requires using mold elements, which are more or less inserts of the manufactured part and which have to be extracted therefrom when the matrix of the composite material of the part is hard.
Quite obviously, the extraction of these elements from molds or cores must be carried out without damaging the part, and this proves to be more or less difficult when the core is relatively enveloped, i.e., enclosed, in the part made of composite material, for example, stiffeners having an Ω-shaped cross section of FIG. 1a. 
In this case, and short of making the stiffeners separately from the skin and proceeding with a later assembly, which is a less satisfactory solution industrially than a simultaneous making of the different components of the panel, the core must be extracted, destroying it or deforming it, because most often the shape of the panel, of the stiffener and of the resulting hollow volume has variations in shape and cross section which make the extraction of the entire core very difficult as FIG. 1b illustrates.
When, because of the shape of the hollow volume such as that corresponding to a stiffener, the core has a very elongated shape, it is difficult to reconcile both the dimensional precision of the core, which is a precision necessary for the precision of the dimensions of the part made, and the stiffness of the core during the making of the part, which is a stiffness that also influences the precision of the part made.
The cores that are destroyed because of being extracted from the part, particularly meltable cores, can be made with good dimensional tolerances, but have the drawback of being heavy and expensive to make, being disposable and most often of having different coefficients of dilatation from those of the composite materials generally used, which makes their use problematic for long shapes, particularly during the making of stiffeners.
The cores entirely made of elastomer, depending on the cross section of the core and the hardness used, either do not have high dimensional stability and are capable of being deformed during the making of the part made of composite material, or do not have striction, and therefore a reduction of cross section, necessary during the removal from the mold, and are removed from the mold with great difficulty with risk of damage to the part made of composite material.
A prior-art solution described in French patent application published under No. 2,898,539 leading to high-quality results consists of making the cores by means of hollow bladders made of silicone.
To make a core, a bladder is made out of silicone with a relatively fine wall closest to the desired shape of the core, in practice in a bladder mold having the shape of the core.
The hollow bladder thus made is placed in a stiff core mold having the desired shape for the core, which it molds by the manufacture itself.
The hollow interior of the bladder is then filled with glass or metal beads and, when the bladder is full of beads, the interior of the bladder is placed under negative pressure, which has the effect of bringing about a compaction of the beads which are blocked in relation to one another without a significant change in the apparent volume of the bladder.
The bladder is then removed from the core mold and forms a core with stable dimensions, corresponding to the dimensions of the core mold, which can be used.
When the part made of composite material is made, the beads are removed from the bladder through an opening and the envelope of the bladder becomes supple and deformable enough to be easily extracted from the part.
The advantage of this method is to be able to form elongated cores, having the stiffness that is needed during the making of the composite part, and being extractable without damaging the part made of composite material. The bladder may be reused several times to make identical cores within the framework of a mass production of parts made of composite material.
A drawback of this method derives from the need to make bladders by molding. In fact, it was discovered by the inventor of the present invention that currently only the molding technique makes it possible to achieve tolerances on the order of a tenth of a millimeter on the dimensions of the cross section of the bladders, a precision necessary for making composite parts of aeronautical quality. This molding technique for bladders becomes a major drawback within the framework of manufacturing a composite part on an industrial scale using cores of very great length. In fact, the molding technique is not suitable for the manufacture of bladders of great length because of a very high risk of nontightness of the bladders, of a noncompliance with the outer shape of the bladders, and of a reduction in the tensile strength.
Molding tools of great length require devices for maintaining the molding air gap creating holes in the bladders, the bladders having to be taken up again to make a sealing of the said holes. End-to-end connection operations of bladder segments to obtain a bladder of great length may also be carried out. The high risks of nontightness are due to the operations of taking up the bladders again and to the end-to-end assembly operations.
Operations of taking up again and/or assembling the molded bladders bring about surface defects of the cores which are the source of markings of the parts made of composite material. These markings of the composite parts may bring about a high rate of rejection of the composite parts.
The operations of taking up again and/or assembling the molded bladders are also sources of local reductions in strength of the bladders and increase the risk of tearing of the bladder during the pulling of the bladder during the removal from mold operation.
For these reasons molded bladders of great length do not meet such industrial requirements as the cost of the cores, compliance with the manufacturing tolerances of the cores and composite parts or the rate of rejection of the composite parts.
Another consequence is the need to make as many models of bladders and bladder molds as shapes of cores to be made.
Hence this core making technique proves to be costly and very risky to use on an industrial scale.